Foundation Sites is a front-end framework for developing responsive websites. Foundation offers various components such as responsive grids, buttons, navigation menus, forms, and typography styles that are easy to customize. It also includes powerful plugins like an image slider, modal dialogs, and an equalizer component that makes it easy to ensure all elements in a row are the same height. Foundation's Sass-powered framework makes it easy to change the look and feel of your website by customizing the variables. Foundation Sites is an excellent choice for building a custom web design system with modular components, ensuring consistency and saving time.
Compared to other popular front-end frameworks like Bootstrap or Materialize, Foundation Sites is more flexible and less opinionated - you can easily organize your custom CSS and SASS however you prefer. On the other hand, since it's less opinionated, it takes a little longer to get started, but with time invested, it is a great choice for a custom-tailored design system.
Alternatives: Bootstrap, Materialize, Semantic UI